Although there is considerable evidence to suggest that host immunocompetence is of great importance in oncogenesis, little is known about the characteristics, and mechanisms of action, of Lymphoid populations operative in immunological surveillance. The impetus for the present study is based upon preliminary observations in our laboratory that subpopulations of lymphocytes undergo changes in their in vivo homing patterns, responses to mitogens, and surface antigens in the spontaneous preleukemic and leukemic states. The lymphoid tissues also show changes in lymphocyte traffic during growth and regression of a virus-induced tumor. The proposed research program is seeking to define the changes which occur in lymphocyte subpopulations associated with spontaneously occurring and experimentally induced neoplasias. These studies will be performed in mice with a high incidence of spontaneous leukemia (AKR/J). Experimental approaches will include evaluation of lymphocyte populations from normal, preleukemic, leukemic and tumor-bearing animals with regard to 1) homing properties as measured by in vivo distribution of Cr-51 labeled cells, 2) surface characteristics, (theta, TL, H-2, Ia, tumor specific and viral antigens, immunoglobulins and complement receptors), 3) response to mitogens (phytohemagglutinin, concanavalin A, pokeweed mitogen and allogeneic cells), 4) stimulatory capacity in mixed lymphocyte reactions.